It's not so much the police that is the problem, as you say, money talks. It is the insurance company if you have an accident, if you are over 90 days the IDP will not be valid, so your UK licence on its own will not be valid either.
I was stopped in a police check in Roi Et a while sgo, they asked for licence and insurance, probably hoping for some T money. As soon as they saw both the licence and IDP they waved me on without bothering to look at either.
I was told (but don't know if it is correct) that it is per entry. I drove for a year on my last one as I was on an out of country non O and had to border bounce every 90 days.
If you take your UK full license + IDP, cert of residence and health certificate along to DLT you wont have to sit the test. You will have to watch the video and take the reaction tests, which TBH are easy. I got both bike and car in a couple of hours.
I had a similar situation to you last December, my B400k would be 2 months in the bank on the same day my extension (non O Thai wife) was due. I went in to the office weeks in advance to talk to them. They told me that I could not get the extn until the following day which would make me one day everstay. The IO advised that I could get a short term extn - visit family - or go OS. His advice was OS 1 day and keep the family visit for any emergency in the future. He said I could only get one per visa.
This was in the Kanchanaburi office, probably different wherever you are, so go in and have a chat
I am in a similar situation to you. I had a ME Non O (Thai wife) visa issued in Suvanalhet. In December last year I extended for 1 year, in country Non O, so no more border bouncing.
Today I went to immigration to ask about changing to retirement and they said as long as I had 12 months of B65k per month deposits from abroad to my Thai bank account, no problem.